1. Field of the Invention
The present invention relates generally to the field of wireless communication. In particular, this invention relates to an active differential mode loop antenna configured to maintain efficient operation across a wide set of use cases for use in wireless communications.
2. Description of the Related Art
The availability of wireless services, such as Global System for Mobile Communications (GSM), Radio Frequency Identification (RFID), Distributed Control System (DCS), Personal Communications Service (PCS), UW, Digital Video Broadcasting-Terrestrial/Handheld (DVB-T/H), Wireless Fidelity (Wifi), Bt, Worldwide Interoperability for Microwave Access (Wimax), Long Term Evolution (LTE), Global Positioning System (GPS), and others, supported by modern handsets, such as MP3 player, mobile phone, laptop, video gaming devices, tablets, and the like have increased significantly during the last decade.
The Numbers of antennas in each device is increasing as well as the number of available wireless services and therefore, the embedded antennas need to be small and require high performance. Modern communication devices such as cellphones typically contain four or five antennas, with each antenna serving a specific function and frequency band. These antennas are closely spaced and are volume constrained, and good isolation between the antennas is needed for efficient operation.
With cellular communication systems becoming more loaded and capacity constrained, the antenna systems on the mobile side of the communication link are expected to become more efficient to assist in maintaining a level of acceptable network performance. Under-performing mobile devices in regard to the radiated performance of the device will degrade the cellular network, with these under-performing devices requiring more system resources compared to more efficient mobile devices.
Several solutions have been proposed over the years to improve the Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) performance of the cellular antenna or to fulfill Specific Absorption Rate (SAR) and Hearing Aid Compatibility (HAC) requirements. Though various antenna techniques and topologies have been proposed and developed to improve antenna efficiency for internal applications, they all suffer from the limitation of being optimized for a single use case such as device in user's hand, device against the user's head, or device in free space environment. To improve on this situation, an antenna can be designed to provide a compromise solution, where the performance of the antenna is considered for a multitude of use cases and is not optimized for a preferred use case.
One antenna structure, called a folded loop antenna, has demonstrated several advantages for handset applications. It can be designed to have several resonances, with one resonance to cover low band cellular frequencies (<1 GHz) and one or multiple resonances to cover high band cellular frequencies (1.5 GHz to 10 GHz bands) when applied to cellular applications. One important benefit of this antenna structure is that one of the different resonances of the folded loop antenna located in the high band (1710 MHZ to 2170 MHZ) is generated from a differential mode (also referred as a balanced mode). The advantages of this differential mode, are lower losses from the head when the phone is in “beside head” position, lower HAC and SAR values.
The differential mode existence is however tightly related to the symmetry of the way the antenna's E and H field are coupling with the mechanics of the host device. A symmetrical radiator design is required to generate the symmetrical coupling, which can be achieved during the antenna design process, but the non-symmetrical mechanical features of the host device will degrade the differential mode. Typically the non-symmetry of the mechanics of the host device is compensated for by introducing non-symmetry in the folded loop antenna radiator pattern.
When a folded loop antenna is designed and integrated into a wireless device for use in Free space conditions, the antenna can be tuned in a such way that the E and H are creating the desired differential mode. However, when the same antenna is used in other use cases such as against the user's head, in the user's hand, surrounded by external objects such as tables, the E and H fields will be disturbed. For example, the antenna performance will be different when the device is against the user's left side of the head as compared to the right side of the head, due to the local environment of the antenna changing between these two use cases when the host device is mobile phone.
Additionally, with the advent of 4G technologies such as LTE (Long Term Evolution) entering service in the mobile wireless industry, there is a need for MIMO (Multiple Input Multiple Output) antenna systems in small mobile devices such as smart phones. For optimal MIMO performance the antenna efficiencies for the two antennas in a MIMO system should be equal. High isolation and low ECC (Envelope Correlation Coefficient) is also required for optimal MIMO antenna system operation, and isolation and ECC can be difficult to achieve in these small form factors. It is difficult to keep the efficiencies of two antennas in a small mobile device equal across the several use cases previously mentioned. The antennas can be designed to provide equivalent performance for a preferred use case, but the efficiencies of the two antennas will diverge as the local environment changes.